Generally, the strongest amount of interest is paid to a portion where fluid flows may possibly leak, causing industrial facilities that deal with dangerous articles, particularly, to suffer a grave accident, so that strict management and construction procedures are needed regarding such a portion or the facilities.
A structure of a representative tube fitting uses a ferrule, and is schematically shown in FIGS. 1 and 2.
As shown in the figure, the tube fitting includes a body 100, a tube 200 which is inserted into the body, a ferrule 300, and a nut 400. Here, the tube fitting can be sealed only by fastening the nut to the extent that the amount of nut rotation reaches a reference value. That is, when the nut 400 is fastened by the amount of nut rotation corresponding to a reference value, the ferrule 300 is moved forward to grip the outer surface of the tube and to be deformed, thereby sealing the tube fitting.
However, when an operator fastens the nut using hand tool, he/she cannot make sure whether the nut has been fastened with precision, and once the fastening has been done, it is impossible for a person other than the operator to check whether the nut has been properly fastened or not.
To solve this problem, at the place of assembly of the tube fitting a proposal has been made to use a special jig, which is designed not to be inserted into a gap between the body and the nut when the nut is not properly fastened, in order to check the fastened state of the nut. That is, if the nut is not properly fastened, the special jig is easily inserted into the gap between the body and the nut. However, problems with this method are that the operator should always carry such a special jig and separately use it at every fastening stage, causing inconvenience, and that a person other than an operator who conducted the fastening work cannot check the fastened state of the nut with a visual inspection.
Further, according to the conventional tube fitting, the hardness of the ferrule is increased over that of the tube so that the ferrule grips the outer surface of the tube in order to seal the tube fitting. However, problems with this configuration are that a variety of fitting conditions must be adhered to.
The fitting conditions required in the conventional tube fitting are as follows. First, the surface of a tube should have no damage such as scratches. This is because if the surface is slightly damaged, leakage may occur through the damaged portion under high pressure.
Second, a tube has a restricted harness. This condition results from the relationship with the ferrule, and the hardness of the ferrule should be greater than that of the tube. This is because the ferrule can be moved so that a leading end thereof is bent to grip the outer surface of the tube only when the hardness of the ferrule is greater than that of the tube and when the nut is fastened to the body. In order to increase the hardness of the ferrule, the ferrule is treated with heat treatment such as carburizing or nitrification. However, if the ferrule is carburized or nitrified, the hardness of the ferrule can be increased, but a problem also occurs in that corrosion resistance is reduced, being a cause of possible leakage.
Third, a tube should have the concentricity within the reference range. Fourth, the thickness of a tube should have a proper value that is not too thick, nor thin. If the thickness of the tube is not proper, a function of the ferrule is not completely formed, and is the cause of reduced airtightness. Fifth, in case of high pressure fitting, the thickness of a tube should be made greater than normal. However, if the thickness is increased, there is a problem of the airtightness being reduced when considering the relationship with the ferrule.
As previously described, the existing fitting structure should satisfy a variety of fitting conditions, so that the fitting is conducted complicatedly along with strict management, which is the cause of an increase in the cost.
For example, a fabricated tube may be carelessly scratched during transportation or treatment of fitting, but the scratched tube cannot be used.